He is Serbian. When Roman meets him off the boat in one of the trailers he says something in serbian to him.

He says "Zdravo Burazeru" Hey Brother, that doesn't mean its Serbian. Thats also Bosnian and Croatian language. And Russian language sounds alike with the Bosnian language. So he must be Russian, or maybe Bosnian/Croatian/Serb.

EDIT: Niko can't be Bosnian/Serb/Croatian. Bellic can't be spelled in Bos/Srb/Cro it can only be spelled with one L. So He is probably something but Bos/Cro/Srb.